Head mounted display systems including an assembly enabling night vision are well known in the art. The prior art as represented in the U.S. Patents, European Patents, European and PCT Patent Applications literature include U.S. Pat. Nos. 4,961,626 to Fournier et al, 5,416,315 to Filipovich, 4,660,943 to Ellis, 4,689,834 to McCarthy et al, 4,775,217 to Ellis, 4,902,116 to Ellis, 5,035,474 to Moss et al, 5,091,719 to Beamon, 5,113,177 to Cohen, 5,184,231 to Ellis, 5,243,450 to Gerbe et al, 5,266,930 to Ichikawa et al, EP Patent no's and PCT 0,284,389 B1 to Evans et al, 0,301,473 B1 to Rotier, and EP Patent Applications, 0,206,324 A2 to Harrison et al, 0,628,261 A1 to Jolly et al, WO 94/14349 to Badou et al, 0,603,027 A1 to Badou et al, 0,603,092 A1 to Perbet et al, 0,599,759 A1 to Lach et al, WO 94/11773 to Fritz et al, 0,579,506 A1 to Bryare, 0,475,790 A1 to Monnier et al, 0,459,580 B1 to Migozzi et al, 0,671,646 A1 to Capdepuy et al, 0,657,111 A1 to Lefort et al, 0,635,742 A1 to Francois et al.
Generally speaking, prior art head mounted displays (HMD), in particular, those providing night/day vision capability are deficient in many respects. Among others, prior art head mounted displays are deficient with respect to: the location of the optical assembly providing night/day vision capability, the types of information available to the viewer, the recording capabilities of the flight and the quality and adaptability of the display for optimum display performance in varying conditions of light intensity. In general, the combination of separate images, not necessarily in a HMD context, is a cumbersome task requiring electronics and signal processing.
A particular problem with prior art head mounted displays is encountered in the case where illuminated symbols are superimposed onto a background of a scene image. The scene image may be viewed directly, as in a daytime image, and in such images, no processing is required to render it acceptable to view.
The scene may also be generated by an indirect image source to enhance the scene image. Generally, indirect imaging is used at night as the image may be poor due to the darkness. An image intensifier (I.sup.2) coupled with a charge coupled device (CCD) camera (ICCD) or forward looking infra-red (FLIR) may be utilized for this purpose. When a night scene is viewed, it is typically viewed with a direct optical coupling system from an image intensifier mounted onto a helmet.
In order to combine the direct day viewing image with the indirect intensified night or symbology scene image, an optical combiner is generally used. An optical combiner is an optical device which enables one to see a single superimposed image from the images of two different objects. An example of such a device is a partially silvered mirror allowing rays from behind it to pass through, whilst reflecting rays incident on it into the same path as the rays arriving from behind.
For purposes of the explanation herein, all references to a direct scene image or direct object refer to an image transmitted without any image processing, and all references to an indirect image refer to an image which has been formed with some type of image processing, such as through an image intensifier, a camera/display, etc.
Reference is now made to FIG. 1A, a basic schematic of a prior art optical combiner 2. Optical combiner 2 receives rays from an image 4 and an image 5. The rays from image 4 are reflected off of the combiner 2, as represented by arrow 3B, and received by an eye 6. The rays from image 5 are transmitted through combiner 2, as represented by arrow 3A, and also received by the eye 6.
Thus eye 6 receives one superimposed image created from the rays 3A from image 5 and from the rays 3B from image 4. The optical equation for the received superimposed image is:
rays 3A'+rays 3B'=superimposed image; PA1 Rays 3A' and rays 3B' are defined as: PA1 rays 3A'=(rays 3A)(T%), where T=% transmission of rays 3A; and PA1 rays 3B'=(rays 3B)(R%), where R=% reflection of rays 3B; PA1 (rays 3A)(T%)+(rays 3B)(R%)=superimposed image, where the superimposed image.ltoreq.100% PA1 T% (transmission ray 3A)+T% (transmission ray 3B).ltoreq.100% or PA1 T% (transmission ray 3C)+T% (transmission ray 3D).ltoreq.100% when T.sub.3A &gt;T.sub.3C and T.sub.3B &lt;T.sub.3D.
The resultant equation after substitutions for 3A' and 3B' is:
Therefore the ratio of mix of rays 3A' and rays 3B' need to be determined in advance. For a prior art optical system however, the resultant image is always less than or equal to 100%, or the sum of the received relative rays 3A and 3B.
The disadvantage of this approach is that the conflicting requirements for image combination during the day and night cannot be catered to in one unit. FIGS. 1B-C which are now referred to are schematic illustrations of the typical day and night biases of the standard prior art combiner system.
FIG. 1B illustrates an example of nighttime transmission of symbology 4 and scene image 5 transmitting two rays 3A-B, through a combiner 2' to an eye 6. FIG. 1C illustrates an example of daytime transmission of symbology 4 and scene 5 transmitting two rays, represented by arrows 3C-D, through a combiner 2" to an eye 6.
As shown in FIG. 1B, during night-use the strength of ray 3A from the scene 5 is required to be greatest due to the poor image produced by the darkness. Conversely, the darkness creates enough contrast for a minimum strength ray 3B from the symbology 4. The two rays 3A and 3B are then combined through combiner 2' with a resultant image received by eye 6 of superimposed symbols 4 on the scene 5.
Conversely, as shown in FIG. 1C, during day-use the strength of ray 3C transmitted from symbology 4 is required to be greater than the strength of ray 3D from the scene 5. This is in order to enhance final superimposed intensity of symbology 4 which may appear faded against a bright day scene 5.
Thus, depending on the external brightness or darkness, the strength of one of the rays 3 from either symbology 4 or from scene 5 is transmitted more, and the other transmission ray 3 is transmitted less. It is noted however, that in all instances for both brightness and darkness, both of the rays are received by combiner in a related proportion of the final image are equal to less than 100% of the final image. Therefore the sum of two rays, as per the equation noted above, is as follows:
There is no simple way to change the transmission/reflection ratio of the optical combiner between day and night in order to accommodate these two differing requirements. Currently, the only viable solution is to alternate between combiner 2' and 2", for either night or day use respectively, whichever is appropriate. However, this solution generally involves changing the whole optical element of the HMD for optimum performance in each situation, hence the requirement for day HMD and night HMD.
Currently, when two different image sources which are looking at a same scene are combined, a single electronic signal processing means is employed. Generally one of the image sources is a Charge Coupled Device (CCD) converting image intensifier (I.sup.2 ) image, and the other image source is a camera converting a direct scene or FLIR. The two image sources are sampled to two separate digital mediums, and the two resultant images are combined pixel by pixel to one digital image using signal processing. The combined digital image is then displayed on a visible image medium (display).
In the particular case of a HMD, such a system may be used either as an integral part of the HMD or as a separate unit. FIG. 2, which is now referred to herein shows a prior art digital sampling and signal processing unit 70 for combining two images in a HMD display. This system is equally applicable to a non-HMD application where two image sources are to be combined. As an example of a HMD combination application, a generated night image is described hereinbelow.
Prior art system 70 consists of an image sensor unit 72, an electronic memory, a memory/signal-processing unit 74 and a display unit 76. Sensor unit 72 contains two or more separate image sources, which may be physically apart.
A first image source 78A may consist of an I.sup.2 generated image and a second image source 78B may consist of a Forward Looking Infra Red (FLIR) image, any other desirable combination, or image sources. The I.sup.2 source 78A is converted to a video signal by a CCD, ICCD, or a camera 80A, and the FLIR source 78B is converted to a video signal by an image converter 80B.
Both "video" signals are respectively sampled and converted to a digital signal by sampler/analog-digital units 82A and 82B respectively. The digital signals are then stored in respective memories 84A and 84B. A signal processor 86 in conjunction with an image processor/timer 88 processes each designated corresponding pixel from memory 84A and 84B and combines them into a single memory 90. The information in memory 90 is converted by a display interface/digital-analog converter 92 and transferred to display unit 76 for display.
A display electronics 94, a display media or source 96 and an eye optics relay 98 display a combined picture for an observer from the data stored in memory 90. An analog image is generated if the display source 96 is a Cathode Ray Tube (CRT). A digital image is generated if the display source 96 is an Active Matrix-Liquid Color Display (AM-LCD), Active Matrix-Electro Luminect (AM-EL), Plasma Display Panel (PDP), flat panel display (FPD) or any other display media.
Unfortunately, the processing unit 74 is cumbersome and some of its elements must be installed off-helmet.